This invention is in the general field of infrared (IR) radiation photography. More particularly it pertains to the high speed photographic recording of far field IR laser beam patterns during the entire period when a high energy laser is operating so that the characteristics of the laser beam over the entire cycle of laser operation can be determined.
There is extensive research being conducted on IR laser systems at the present time and one of the areas under continuous investigation is how to improve the evaluation of the various characteristics of the laser beam. This evaluation is often referred to as beam diagnosis. Some of the beam characteristics to be diagnosed or determined are far field beam intensity, near field phase, jitter and absolute power of the beam. The usual approach to monitoring IR beams for these parameters is to use electronics intensive electro-optic techniques requiring very expensive photodetectors, recorders and signal processing equipment. Since these techniques are expensive and time consuming to use, other simpler approaches are needed.
It has been suggested in the literature by Frazier et al., Applied Optics/Vol. 15, No. 6/June 1976, that IR photography might be attractive for photography of far-field high energy laser mode patterns. Frazier discloses in U.S. Pat. No. 4,018,608 a process for photographing IR laser beam patterns on silver halide film by first impinging an IR laser beam on the film and then flashing the film with visible light. Both of the Frazier publications identified above contain a discussion of IR presensitization photography. IR sensitization theory is also discussed by Naor et al. in Applied Optics/ Vol 20, No. 14/15 July 1981. Naor et al. theorizes that the sensitization of the photographic film results from the heating produced in the photographic emulsion by the IR radiation. Naor et al. also explains that the film may be sensitized or desensitized depending upon the duration of the visible exposure that follows the IR exposure. Naor et al. further reports that sensitivity effects were improved by delays shorter than 0.5 msec between the IR and visible exposure. The basic approach used in all of the investigations discussed above, and in the invention disclosed in this application, utilizes direct photographic recording of IR radiation on silver halide film. Two sequential exposures are made, an IR exposure to sensitize a portion of the film, followed immediately by a uniform visible exposure of all the film in the frame. After development those areas of the film that have been exposed to IR and visible radiation are darker than the areas that have only been exposed to visible radiation.